Hollow fibres

ABSTRACT

A hollow fibre is provided possessing a continuous longitudinally extending channel free from macromolecular material, said fibre consisting essentially of a copolymer of acrylonitrile and an olefinically unsaturated comonomer containing an optionally salified sulphonic acid group, and possessing micropores of average diameter less than about 100 A, between 40 and 80% of walls of the fibre being empty space. This fibre can be prepared by injecting a solution of a copolymer of acrylonitrile and an olefinically unsaturated comonomer containing an optionally salified sulphonic acid group in at least one polar organic solvent into a spinneret with an annular orifice, and immediately coagulating the inside and the outside of the nascent hollow fibre issuing from the spinneret with a coagulating fluid which is selected from: A. A LESS THAN SATURATED AQUEOUS SOLUTION OF AN INORGANIC SALT, SAID SOLUTION OPTIONALLY CONTAINING UP TO 40% OF A MISCIBLE POLAR ORGANIC SOLVENT, AND B. AT LEAST ONE POLAR ORGANIC SOLVENT WHICH IS A NON-SOLVENT FOR THE ACRYLONITRILE COPOLYMER AND IS MISCIBLE WITH THE SOLVENT FOR THE COPOLYMER, THE COAGULATION INSIDE THE FIBRE BEING CARRIED OUT BY INJECTING COAGULATING FLUID INSIDE THE NASCENT FIBRE. Such fibres possess excellent permeability to water which makes them particularly useful in ultrafiltration as well as dialysis, especially haemodialysis.

United States Patent Christen et al.

[ Dec. 30, 1975 HOLLOW FIBRES [75] Inventors: Gilbert Christen, Lyon;Bernard Favre, Ecully; Xavier Marze, Lyon; Michel Salmon, Mions; ReneThuillier, Morance, all of France [73] Assignee: Rhone-Poulenc, S.A.,Paris, France [22] Filed: Sept. 7, 1973 [2]] Appl. No.: 395,155

[30] Foreign Application Priority Data Sept. 12, 1972 France 72.32285[52] US. Cl 428/398; 210/321; 210/500 R; 210/505; 264/41; 264/182 [51]Int. Cl. B05D 3/00; D026 3/22 [58] Field of Search 161/178, 177;264/182,

[56] References Cited UNITED STATES PATENTS 3,674,628 7/1972 Fabre161/178 3,701,820 10/1972 Kuratani et al. 264/182 PrimaryExaminer-Lorraine T. Kendell Attorney, Agent, or FirmStevens, Davis,Miller & Mosher 57 ABSTRACT A hollow fibre is provided possessing acontinuous longitudinally extending channel free from macromolecularmaterial, said fibre consisting essentially of a copolymer ofacrylonitrile and an olefinically unsaturated comonomer containing anoptionally salified sulphonic acid group, and possessing micropores ofaverage diameter less than about 100 A, between 40 and 80% of walls ofthe fibre being empty space. This fibre can be prepared by injecting asolution of a copolymer of acrylonitrile and an olefinically unsaturatedcomonomer containing an optionally salified sulphonic acid group in atleast one polar organic solvent into a spinneret with an annularorifice, and immediately coagulating the inside and the outside of thenascent hollow fibre issuing from the spinneret with a coagulating fluidwhich is selected from:

a. a less than saturated aqueous solution of an inorganic salt, saidsolution optionally containing up to 40% of a miscible polar organicsolvent, and

b. at least one polar organic solvent which is a non-solvent for theacrylonitrile copolymer and is miscible with the solvent for thecopolymer,

the coagulation inside the fibre being carried out by injectingcoagulating fluid inside the nascent fibre. Such fibres possessexcellent permeability to water which makes them particularly useful inultrafiltration as well as dialysis, especially haemodialysis.

8 Claims, 1 Drawing Figure US. Patent Dec. 30, 1975 3,930,105

HOLLOW FIBRES dia of Polymer Science and Technology, 15, 258-272l-lollow fibres made of cellulosic esters such as cellulose acetate havebeen developed in particular; nevertheless, such fibres possessdisadvantages. Amongst these disadvantages, there may be mentioned'th-erisk of changes in the properties of the fibres as aresult-of slowand/or partial hydrolysis of the ester groups in-the cellulose, thishydrolysis taking place either I simply under atmospheric conditionsover a period of time, or, for example, under the effect of cleaningagents. This is why attempts have been made to produce hollow fibresbased on other materials; for this purpose acrylonitrile polymers haveproved particularly valuable.

In parallel with this development regarding the chemical nature of thehollow fibres, investigations have been directed to making the structureof the fibres more suitable. In particular, hollow fibres have beensought which have improved permeability towards fluids which passthrough them. Thus, in U.S. Pat. No. 3,423,491, it has been proposed tomelt-spin a mixture of a thermoplasticpolymer and a plasticiser, andthen to remove the plasticiser by lixiviation. The thermoplastic polymercan be a polymer of acrylonitrile and a non-ionic comonomer such asvinyl acetate. The plasticiser content in the mixture is of courselimited by the fact that it must still be possible to melt-spin thismixture. in fact, hollow fibres prepared in this way are mainlyeffective for use in reverse osmosis because they possess a degree ofsalt rejection which is, in practice, greater than 75%. v

For certain applications such as ultrafiltration and dialysis,especially haemodialysis, hollow fibres which have such a degree ofrejection of salt are undesirable and may even be forbidden.

It has also been proposed to produce hollow fibres with a skin made ofacrylonitrile polymers, the wall of these fibres consisting essentiallyof a skin and a porous substrate. These fibres are described in US. Pat.No. 3,674,628. Theyv have also been described in the research anddevelopment report of the USA. Government No. PB.l92,846. The processfor their preparation consists of a. injecting a polymer solution into aspinneret with a ring-shaped or annular orifice,

b. solidifying a peripheral zone of the filament issuing from thespinneret, and then simultaneously or subsequently,

c. coagulating the internal and/r external peripheral zone of the fibre.In practice, the solidification is carried out by passing thefibre as itis formed through air at the outlet of the spinneret which is aso-called emergentspinneret; the coagulation is effected by the actionofa coagulating agent, that is to say a non-solvent for the polymer ofwhich the fibre is made. I

A similar process has also been proposed for cellulosic fibres (seeEnc-ycl. Polym. Sc. Tech., 15, 263 (1971)), but it produces fibres forreverse osmosis which have high degrees of rejection of salts. It is ineffectknown, as us. Pat. No. 3,423,491 confirms, that the way in whichthe fibres are prepared generally has a great influence on theproperties and/or performance of the fibres.

Although such fibres possessing a skin provide considerable advantages,they nevertheless possess disadvantages in certain cases; thus, when itis desired to improve the turbulence of the fluids passing throughapparatuses possessing hollow fibres by giving these fibres particularshapes such as waves, curls and braidings, the'skin of such fibrespresents a point of weakness which can give rise to stray fluid flows.The fragility of the fibres also presents problems in other situations,for example when it is desired to handle them in automatic apparatuseswithhigh flow rates and with great accelerations or decelerations andwhen they are used with a differential pressure between the walls of thefibres. In the latter case, which applies in ultrafiltration, it is alsofound that the hollow fibres are the more fragile, the higher is thecontent of ionic comonomer in the acrylonitrile polymer; difficultiescan arise when, for example, greater than 5% (by number) of therecurring units are derived from the ionic comonomer.

An aim of the present invention is to provide hollow fibres made ofacrylonitrile polymer which do not have the disadvantages of the priorart, and, especially, to provide fibres which do not possess a skin butwhich possess high permeability.

According to the present invention, there are provided hollow fibrescharacterised in that:

1. they consist of a copolymer of acrylonitrile and an olefinicallyunsaturated monomer carrying optionally salified sulphonic acid groups(this monomer being denoted hereafter as the sulphonic acid comonomer),

2. they are microporous and possess micropores of average diameter lessthan 100 A, and 3. between 40 and of the walls is empty space. 'Theproportion of empty space" in the wall of a microporous hollow fibre, isdefined by V, being the volume occupied by the wall of a sample E, ofmicroporous hollow fibre of weight p, and V being the volume occupied bythe wall ofa sample E of hollow fibre with a compact structure having aweight p and made of the same polymeric material as the microporoushollow fibre.

By hollow fibre with a compact structure, there is meant a hollow fibrewhich is watertight under an internal relative pressure of 3 bars.

ln practice, the volumes V, and V can be calculated from measurements ofthe length and the internal and external diameters of the samples E, andE these measurements being themselves made by simple opticalobservations (with a microscope in the case of the diameters). Thesample E of hollow fibre with a compact structure of volume V, can beobtained by drying, for example at 60C and under an absolute pressure of10 mm of mercury, the sample E, of microporous hollow fibre of volumeV,; although the dimensions of the sample of hollow fibre vary duringdrying due to shrinkage, nevertheless the weight p of dry material isretained and the volume V, of the sample E is the same 3 (preciselybecause of its compact structure) as the volume of an imaginary sample Ehaving the same length and the same internal diameter as IE anddiffering from IE only by its external diameter; it follows that theproportion of empty space corresponds to the volume shrinkage due to thechange from the microporous structure to the compact structure.

Products of the formula: CHR CR AY(I) in which Y represents a SO H or SOM group, M being a metal atom, preferably an alkali metal atom, R and Rindependently represent a hydrogen atom or a methyl group, A representsa valency bond or a group A' or O--A', in which A represents a straightor branched, saturated or unsaturated, divalent aliphatic hydrocarbongroup, an unsubstituted aromatic nucleus, or a monoaromaticmonoaliphaticchain in which one of the free valencies is carried by an aliphaticcarbon atom and the other by a carbon atom of the aromatic nucleus, aregenerally used as the sulphonic acid comonomer of acrylonitrile.

Specific sulphonic acid comonomers which may be used includevinylsulphonic, allylsulphonic, methallylsulphonic, styrenesulphonic,vinyloxybenzenesulphonic, allyloxyand methallyloxybenzenesulphonic, andallyloxyand methallyloxyethylsulphonic acids, as well as the salts ofthese various acids, preferably their alkali metal salts.

The proportion of sulphonic acid comonomer in the acrylonitrilecopolymer is generally between 1 and 50% (by number) of sulphonic acidmonomer units, and preferably between 5 and l5%. The acrylonitrilecopolymers have a specific viscosity (measured at 25C as a 2g/l solutionin dimethylformamide) which is usually between 0.1 and 3, and preferablybetween 0.5 and 1.5.

The diameter of the micropores in the fibres of the present inventioncan be determined with the aid of an electron microscope, for exampleusing a magnification of 20,000 by observing the surfaces and/or thecrosssections of the fibres.

The external diameter of the hollow fibres of this invention is usuallybetween 50 and 1,000 ;4., and preferably between 100 and 600 t; thethickness of the walls is generally between 5 and 40%, and preferablybetween and 25%, of the external diameter.

The hollow fibres according tq j 'the invention are generally free fromvacuoles i.e. empty spaces present in the walls of the fibres, thegreatest dimension of such spaces being greater than approximately 5 u,and do not possess a skin or a dense layer at the surface. Their degreeof salt rejection is generally zero, measured under a pressure of 2 barsfor an aqueous solution containing l0 g/l NaCl.

The present invention also relates to a process for the preparation ofhollow fibres, especially of hollow fibres as defined above. Thisprocess is characterised in that a collodion, formed from a solution ofthe copolymer of acrylonitrile and the sulphonic acid comonomer in apolar organic solvent (or mixture of solvents) is injected into aspinneret with an annular orifice, and that, immediately at the outletof the spinneret, the inside and the outside of the nascent hollow fibreare coagulated by means of a coagulating fluid which is either:

4 a. an aqueous solution of an inorganic salt of concentrations lessthan that required for saturation, usually between 1 and 35% by weight,and preferably between 5 and 20%, or 5 b. a polar organic solvent or amixture of polar organic solvents, this solvent (or mixture of solvents)being a non-solvent for the acrylonitrile copolymer and being misciblewith the solvent for the collodion.

By collodion is meant a solution of the copolymer of acrylonitrile andthe sulphonic acid comonomer such that it can be spun.

Known solvents for the copolymers of acrylonitrile and the sulphonicacid monomer are generally used as 15 the polar organic solvent which iscapable of forming the collodion, in particular dimethylsulphoxide, di-

methylacetamide, hexamethylphosphotriamide and, especially,dimethylformamide (DMF).

Instead of a single solvent, the collodion can be produced from amixture of solvents; it is also possible to add a minor amount of anon-solvent for the polymer to this solvent or mixture of solvents,insofar as the whole remains a solvent for the acrylonitrile polymer.

The concentration of acrylonitrile copolymer in the collodion isgenerally greater than 5% by weight and less than that required forsaturation; it is preferably greater than 20% by weight.

The collodion is then coagulated by simply bringing the collodion intocontact with the coagulating fluid, immediately at the outlet of thespinneret.

Such a coagulation has been disclosed in U.S. Pat. No. 3,674,628 but inthe process described there is a sufficient temperature differencebetween the collodion and the coagulating fluid for the fibre to setimmediately it leaves the spinneret. In the process of this invention,on the other hand, there is no such setting step and the difference intemperature between the collodion and the coagulating fluid is generallyless than 30C.

The external coagulation of the nascent hollow fibre is effected inpractice by making thefibre, during its formation, to flow through abath of coagulating fluid (or coagulating bath).

The internal coagulation of the nascent hollow fibre is effected inpractice by injecting coagulating fluid into the core, that is to sayinto the inside of the fibre during its formation.

The temperature of the coagulating fluids and of the collodion can varywithin wide limits; they are generally between l0 and +40C, andpreferably between 0 and 30C. Low temperatures generally favour theabsence of vacuoles. The temperatures of the collodion, of the internalcoagulating fluid and of the spinneret are usually the same for economicand technical reasons; in contrast, the temperature of the coagulatingbath can be different from the other three.

When the coagulating fluid is an aqueous solution of an inorganic salt,a salt of an alkali metal or alkaline earth metal which is soluble inwater is advantageously used; it is generally preferred to use sodiumchloride. It is however also possible to use lithium, sodium, potassium,magnesium and calcium chlorides, sulphates, nitrates and perchlorates,within the limits of their solubility.

The non-solvent power of these aqueous solutions can be altered byadding miscible polar organic solvents, for example dimethylformamide,in a proportion which is preferably less than 40% (by volume).

When the coagulating fluid is a polar organic solvent or a mixture ofpolar organic solvents, alcohols such as methanol, ethanol, propanolsand butanols, aliphatic diols, especially ethylene glycol, or aliphaticketones known as acetone and methyl ethyl ketone, are advantageouslyused as the non-solvent for the acrylonitrile copolymer; the non-solventpower of this solvent or mixture of solvents can be changed by addingminor amounts (generally less than 25%) of solvents for theacrylonitrile copolymer such as those mentioned above e.g.dimethylformamide, dimethylsulphoxide, dimethylacetamide andhexamethylphosphotriamide.

During the coagulation, a part of the solvent for the collodion migratesinto the coagulating fluid and this, consequently, can change thecomposition thereof somewhat.

For the purpose of ensuring that the hollow fibres have a uniform andsymmetrical shape, it is preferred to position the spinneret along avertical axis with the collodion flowing in a downwards direction; inpractice, the spinnerets are immersed spinnerets.

The nascent fibre is kept in contact with the coagulating fluids atleast until the fibre is 'sufficiently hardened to enable it to behandled and no longer flows under the working conditions.

The coagulation described above can be followed by washing with purewater in order to remove nonpolymeric constituents from the fibre(especially so]- vents and/or salts).

The fibres, prepared by the coagulation process described above, can begiven an aqueous treatment for the purposes of improving theirperformance, especially their permeability.

The hollow fibres subjected to this treatment can have undergone apartial washing with pure water or with a mixture of water and anorganic solvent but, regardless of this, at the time of the aqueous heattreatment, the fibres still advantageously contain a little of thesolvent or solvents which initially formed the collodion; moreprecisely, the proportion of residual solvent in the hollow fibresduring the aqueous heat treatment is generally between 5 and 20%, andpreferably between and 17%.

This aqueous heat treatment generally consists of immersing the hollowfibres in water or in a mixture of water and a non-solvent at atemperature between 60 and 250C, and preferably between 80 and 190C.

The water or the aqueous mixtures used can be in the vapour phase;however, it is preferable to use them in the liquid phase. Of course, atreatment above 100C can make it necessary to work under pressure whenit is desired to use liquid water in order to carry out the aqueous heattreatment.

The proportion of water in the aqueous mixtures which can be used inthis treatment is usually greater than 50% by weight, and preferablygreater than 90%. The water can be mixed with organic solvents or withinorganic or organic electrolytes but is then preferred to use mixtureswhich are chemically neutral and especially not strongly basic so as toavoid chemical attack of the acrylonitrile copolymer. A pH of 6 to 8 isgenerally suitable.

According to an advantageous procedure, the aqueous heat treatment iscarried out continuously by making the fibre flow continuously throughthe hot water treatment bath, the pressure being atmospheric and thetemperature being at most equal to 100C. The dura- 6 tion of thetreatment is usually 5 seconds to 5 minutes but there is no criticalupper limit.

The aqueous heat treatment described above is preferably accompanied bystretching the fibres longitudinally; this stretching is usually 50 to500% and preferably to 250%.

Finally, it is possible to impart better dimensional stability to thestretched fibres by carrying out a relaxation by subsequently leavingthe fibres, without a stretching force, in an aqueous bath at atemperature which is preferably below that of the heat treatment.

The fibres according to the invention can be stored in the moist state,particularly using glycerine, for example immersed in a mixture of waterand glycerine containing at least 40% by weight of glycerine.

The fibres according to the invention possess excellent permeability towater, which makes them especially advantageous in ultrafiltration aswell as in dialysis, especially haemodialysis. They do not clog readilyand they are very suitable for separating macromolecular solutions; theycan thus be used in a variety of ultrafiltration applications. They alsohave a good resistance to pressure.

Amongst the applications of the hollow fibres according to the invention, the production of enzymatic reactors should 'be mentioned inparticular. Such a reactor can be equipped as shown diagrammatically inthe FIGURE of the accompanying drawings.

A reactor 1 comprises a plurality of hollow fibres shown generally at 2and a system of compartments which makes it possible to achieve twoliquid flows, one inside the hollow fibres and the other outside. Theliquid flowing outside the hollow fibres passes successively through acompartment 3, pipeline 4, expansion vessel 5, pipeline 6 and pump 7;the liquid flowing inside the hollow fibres passes successively throughcompartment (8), pipeline (9), expansion vessel 10, pipeline ll, pump12, compartment 13 and fibres 2.

In using such a reactor, the enzyme, in solution or suspension, is madeto flow on one side of the walls of the hollow fibres and a substrate(in solution or suspension) is made to flow on the other side of thesesame walls. For the purposes of convenience and simplicity, the enzymewill be described as flowing outside the hollow fibres and the substrateinside, it being understood that the reverse is perfectly possible.Thus, in the apparatus of the FIGURE, the enzyme passes through thecircuit 3, 4, 5, 6 and 7, and the substrate passes through the circuit8, 9, l0, l1, l2, l3 and the channel inside the fibres.

The enzyme/substrate pairs which are capable of being reacted in theenzymatic reactors are such that the hollow fibres are practicallyimpermeable to the enzyme and permeable to the substrate and to theproducts of the enzymatic reaction; in this way, it is easy to separatethe enzyme from its reaction mixture (substrate reaction products) andlosses of enzymes are minimal.

The hollow fibres according to the invention make it possible inparticular to treat urea with urease, and hence they are used inartificial kidneys.

The following Examples further illustrate the present invention. Allthehollow fibres of these Examples have zero salt rejection.

EXAMPLE 1 A collodion is prepared by dissolving 23.5 g of a copolymer ofacrylonitrile and sodium methallylsul- 7 phonate (9% by weight ofmethallylsulphonate, corresponding to 3.2% by number of sulphonic acidmonomer units; specific viscosity measured at C as a 2g/l solution inDMF 0.90) in 76.5 g of dimethylformamide (DMF).

This collodion is injected at the rate of 4.5 em /minute into theannular (ring-shaped) orifice ofa spinneret (internal diameter of thering: 0.6 mm; external diameter of the ring: 0.8 mm).

The spinneret is arranged along a vertical axis and its lower orificeend is immersed in an aqueous solution of sodium chloride ofconcentration 200 g/l at 2C. At the centre of the annular orifice of thespinneret, there is a second orifice of diameter 0.3 mm, through whichan aqueous solution of sodium chloride of concentration 200 g/l, at 23C,is injected, at the rate of 1.7 em /minute, into the core of the nascenthollow fibre.

The nascent hollow fibre travels vertically downwards through thecoagulating bath over a length of 1 m at the rate of 9 m/minute; at theoutlet of this coagulating bath, the hollow fibre passes horizontallyfor cm through a bath of boiling water and it is guided at the inlet andat the outlet of this bath by wheels, the speed at which the fibreenters this bath being 9 m/minute and the speed at which it issues being27 m/minute. At the outlet, the fibre is wound up on a roller; thetreatment of the fibre is completed by washing with pure water bysprinkling it for 1 minute over turns wound up on the roller (the fiberswere thus stretched in a ratio of 3).

The fibre thus produced has an internal diameter of 350 u, an externaldiameter of 520 p. and a proportion of empty space (in the walls) or61%; a study of crosssections of this fibre under an electron microscope(magnification: 20,000) shows that it does not have any pores ofdiameter greater than or equal to 100 A.

From the fibre thus prepared, an ultrafiltration apparatus (or module)is produced which possesses 180 fibres of length cm, each with a usefullength of 64 cm, arranged in parallel (64 cm represent the lengthavailable for ultrafiltration). The apparatus thus has an averagesurface area for exchange of 0.13 m.

An aqueous solution, of concentration 1 g/l, of bovine albumin ofmolecular weight 70,000, is made to flow outside these fibres, under arelative pressure of 2 bars.

An untrafiltrate is obtained with a flow rate of 121 l/day, m and adegree of rejection of 100% [the degree of rejection is defined byexpression:

concentration of solute in the ultrafiltrate 8 Other characteristics ofthese fibres are given in Table I, together with characteristics of thefibres of Examples 2 to 7.

EXAMPLE 2 Example 1 is repeated, changing the stretching ratio in theboiling water; the fibre is stretched to 1.5 times its original length.I

The hollow fibre obtained has an internal diameter of 370 y. and anexternal diameter of 660 a. The proportion of empty space is 54%. Thereare not pores of diameter greater than or equal to 100 A.

EXAMPLE 3 Example 1 is repeated, changing the stretching ratio inboiling water; the fibre is stretched to 5 times its original length.

The hollow fibre obtained has an internal diameter of 255 u and anexternal diameter of 385 u. The proportion of empty space is 56%. Thereare no pores of diameter greater than or equal to 100 A.

EXAMPLE 4 Example 1 is repeated, dispensing with the treatment usingboiling water.

EXAMPLE 5 EXAMPLE 6 Example 5 is repeated, using an external coagulatingbath at 0C. The fibres have a proportion of empty space of 57%, anexternal diameter of 450 u and an internal diameter of 280 ;1.. Thereare no pores of diameter greater than or equal to 100 A.

EXAMPLE 7 Example 6 is repeated, replacing the water of the externalcoagulating bath with methanol and dispensing with the salt (NaCl). Thefibres have a proportion of empty space of 69%, an external diameter of435 p. and an internal diameter of 275 t. There are no pores of diametergreater than or equal to 100 A.

Some characteristics of the fibres prepared in the preceding Examplesare given in Table I.

TABLE 1 Ultrafiltration of pure water under a relative Ultrafiltrationof a solution containing 1 g/l of a macromolecular solute at 23C under arelative pressure of 2 bars outside the pressure of 2 bars fibres.Example inside the fibres. Nature of Molecular weight Flow rate of theDegree of Flow rate in l/day. m the solute of the solute ultrafiltratcin rejection at 23C. l/day. m in l 324 bovine albumin 70,000 121 I00 1dextran 40,000 I l l 1 lysozyme I5 .000 85 I00 2 283 3 240 4 bovinealbumin 70,000 47 5 513 dcxtrun 40.000 277 41 5 ovalbumin 45,000 279 1006 557 dextran 40,000 204 57 TABLE I-continued Ultrafiltration of pureUltrafiltration of a solution containing 1 g/l of a macromolecular waterunder a relative solute at 23C under a relative pressure of 2 barsoutside the pressure of 2 bars fibres. Example inside the fibres. Natureof Molecular weight Flow rate of the Degree of Flow rate in llday. m thesolute of the solute ultrafiltrate in rejection at 23C. 1/day. m in 7727 dextran 40,000 128 39 7 ovalbumin 45,000 131 95 Furthermore, nourease is observed in the circuit assin throu h the inside of thefibres. EXAMPLE 8 p g g A collodion, prepared as in Example 1, isinjected at the rate of 9.4 cm /minute into the annular orifice of aspinneret similar to that of Example 1.

The spinneret is arranged along a vertical axis and its lower orificeend is immersed in an aqueous solution of NaCl of concentration 200 g/lat 8C.

At the centre of the annular orifice of the spinneret, there is a secondorifice of diameter 0.3 mm, through which an aqueous solution of NaCl ofconcentration 200 g/l, at 23C, is injected at the rate of 1.16 em/minute into the core of the nascent hollow fibre.

The nascent hollow fibre travels through the coagulating bath over alength of 1 m at a speed of 12 m/minute.

At the outlet of this coagulating bath the hollow fibre passes for 30 cmthrough a bath of boiling water, the

"pr oduced which possesses 300 fibres, each of 72 cm,

arranged in parallel (72 cm represent the length available forultrafiltration). This apparatus is used in an installation as describedin FIG. 1.

1 Litre of an aqueous solution of urea of concentration g/l is made toflow inside the fibres at a flow rate of 10.5 l/hour. 0.5 L of anaqueous solution of urease of concentration g/l is made to flow outsidethe fibres at a flow rate of 7.5 l/hour. The temperature of thesolutions is 30C.

In order to determine the rate of conversion of urea, the ammoniumcarbonate formed in the urea solution is measured periodically (usingN/lO HCl; indicator: Methyl Orange). From the result of thismeasurement, the corresponding amount of urea which has been convertedis calculated, without taking account of the ammonium carbonate presentin the urease circuit in this calculation.

After 15 minutes, the urea is converted at the rate of 0.8 g/l. After 45minute, the urea is converted at the rate of 1.85 g/l (by way ofcomparison, if the procedure followed had been simple mixing, the ureawould have been converted respectively at the rate of 1.5 and 3.5 g/

We claim:

1. A skinless hollow fibre possessing a continuous longitudinallyextending channel free from macromolecular material, said fibreconsisting essentially of a copolymer of acrylonitrile and olefinicallyunsaturated comonomer containing an optionally salified sulphonic acidgroup, the proportion of optionally salified sulphonic acid groups insaid copolymer being 1 and 50% (by number) of the monomer units andpossessing micropores of average diameter less than about 100 A, between40 and of walls of the fibre being empty space, said fibre possessingzero salt rejection, measured under a pressure of 2 bars for an aqueoussolution containing 10 g/l of sodium chloride.

2. A hollow fibre according to claim 1 in which the sulphonic acidcomonomer has the formula: CHR CR AY in which Y represents a SO H or SOM group, M being a metal atom, R and R independently represent ahydrogen atom or a methyl group, A represents a valency bond or a groupof formula A or OA', in which A represents a straight or branched,saturated or unsaturated, divalent aliphatic hydrocarbon group, anunsubstituted aromatic nucleus or a monoaromaticmonoaliphatic chain inwhich one of the free valencies is carried by an aliphatic carbon atomand the other by an aromatic carbon atom.

3. A hollow fibre according to claim 2 in which M represents an alkalimetal.

4. A hollow fibre according to claim 1 in which the sulphonic acidcomonomer is selected from vinylsulphonic, allylsulphonic,methallylsulphonic, styrenesulphonic, vinyloxybenzenesulphonic,allyloxyand methallyloxy-benzenesulphonic and allyloxyandmethallyloxyethylsulphonic acid and a salt thereof.

5. A hollow fibre according to claim 1 in which the proportion ofsulphonic acid monomer units is between 5 and 15%.

6. A hollow fibre according to claim 1 in which the acrylonitrilecopolymer has a specific viscosity (measured at 25C as a 2 g/l solutionin dimethylformamidc) of between about 0.1 and 3.

7. A hollow fibre according to claim 1 which has an external diameter ofbetween about 50 and 1,000 n, and a wall thickness of between about 5and 40% of the external diameter.

8. A hollow fibre according to claim 7 which has an external diameter ofbetween about and 600 p, and a wall thickness between about 10 and 25%of the external diameter.

1. A SKINLESS HOLLOW FIBRE POSSESSING A CONTINUOUS LONGITUDINALLYEXTENDING CHANNEL FREE FROM MACROMOLECULAR MATERIAL, SAID FIBRECONSISTING ESSENTIALLY OF A COPOLYMER OF ACRYLONITRILE AND OLEFINICALLYUNSATURATED COMONOMER CONTAINING AN OPTIONALLY SALIFIED SULPHONIC ACIDGROUP, THE PROPORTION OF OPTIONALLY SALIFIED SULPHONIC ACID GROUPS INSAID COPOLYMER BEING 1 AND 50% (BY NUMBER) OF THE MONOMER UNITS ANDPOSSESSING MICROPORES OF AVERAGE DIAMETER LESS THAN ABOUT 100 A, BETWEEN40 AND 80% OF WALLS OF THE FIBRE BEING EMPTY SPACE, SAID FIBREPOSSESSING ZERO SALT REJECTION, MEASURED UNDER A PRESSURE OF 2 BARS FORAN AQUEOUS SOLUTION CONTAINING 10 G/L OF SODIUM CHLORIDE.
 2. A hollowfibre according to claim 1 in which the sulphonic acid comonomer has theformula: CHR1 CR3-A-Y in which Y represents a -SO3H or -SO3M group, Mbeing a metal atom, R1 and R3 independently represent a hydrogen atom ora methyl group, A represents a valency bond or a group of formula -A''-or -O-A''-, in which A'' represents a straight or branched, saturated orunsaturated, divalent aliphatic hydrocarbon group, an unsubstitutedaromatic nucleus or a monoaromatic-monoaliphatic chain in which one ofthe free valencies is carried by an aliphatic carbon atom and the otherby an aromatic carbon atom.
 3. A hollow fibre according to claim 2 inwhich M represents an alkali metal.
 4. A hollow fibre according to claim1 in which the sulphonic acid comonomer is selected from vinylsulphonic,allylsulphonic, methallylsulphonic, styrenesulphonic,vinyloxybenzenesulphonic, allyloxy- and methallyloxy-benzenesulphonicand allyloxy- and methallyloxy-ethylsulphonic acid and a salt thereof.5. A hollow fibre according to claim 1 in which the proportion ofsulphonic acid monomer units is between 5 and 15%.
 6. A hollow fibreaccording to claim 1 in which the acrylonitrile copolymer has a specificviscosity (measured at 25*C as a 2 g/l solution in dimethylformamide) ofbetween about 0.1 and
 3. 7. A hollow fibre according to claim 1 whichhas an external diameter of between about 50 and 1,000 Mu , and a wallthickness of between about 5 and 40% of the external diameter.
 8. Ahollow fibre according to claim 7 which has an external diameter ofbetween about 100 and 600 Mu and a wall thickness between about 10 and25% of the external diameter.